Image encoding apparatus adaptively determining a scan pattern according to an intra prediction mode

ABSTRACT

Provided is an image encoding apparatus that generates a prediction block according to an intra prediction mode, generates a residual block, transforms the residual block to generate a transform block, quantizes the transform block to generate a quantized transform block, scans the quantized transform block to generate one-dimensional (1D) quantized coefficients, and entropy-codes the 1D quantized coefficients. When a size of the transform block is 8×8, the quantized transform block is divided into a plurality of sub-blocks, the plurality of sub-blocks are scanned and coefficients of each sub-block are scanned using a scan pattern determined according to the intra prediction mode. The scan pattern for scanning the plurality of sub-blocks is the same as the scan pattern for scanning the coefficients of each sub-block and when the intra prediction mode is a horizontal mode, the scan pattern is a vertical scan.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 13/624,826 filed on Sep. 21, 2012, which is a continuationapplication of International Application No. PCT/KR2011/005592 filed onJul. 29, 2011, which claims priority to Korean Application No.10-2010-0074462 filed on Jul. 31, 2010 and Korean Application No.10-2011-0062603 filed Jun. 28, 2011, which applications are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to an image encoding apparatus, and moreparticularly, to an apparatus of generating a prediction block capableof minimize the amount of coding bits of a residual block.

BACKGROUND ART

In image compression methods such as Motion Picture Experts Group(MPEG)-1, MPEG-2, MPEG-4 and H.264/MPEG-4 Advanced Video Coding (AVC),one picture is divided into macroblocks to encode an image. Then, therespective macroblocks are encoded using inter prediction or intraprediction.

In intra prediction, a current block of a current picture is encoded notusing a reference picture, but using values of pixels spatially adjacentto the current block. An intra prediction mode with little distortion isselected by comparing a prediction block generated using the adjacentpixel values with an original macroblock. Then, using the selected intraprediction mode and the adjacent pixel values, prediction values of thecurrent block are calculated. Difference between the prediction valuesand pixels values of the original current block are calculated and thenencoded through transform coding, quantization and entropy coding. Theintra prediction mode is also encoded.

Intra prediction are generally classified into 4×4 intra prediction, 8×8intra prediction and 16×16 intra prediction for luminance components andchrominance components.

In 16×16 intra prediction according to related art, there are four modesof a vertical mode, a horizontal mode, a direct current (DC) mode and aplane mode.

In 4×4 intra prediction according to related art, there are nine modesof a vertical mode, a horizontal mode, a DC mode, a diagonal down-leftmode, a diagonal down-right mode, a vertical right mode, a vertical leftmode, a horizontal-up mode and a horizontal-down mode.

Each prediction mode has indexed according to the frequency of use ofthe respective modes. The vertical mode that is mode 0 shows the highestpossibility of being used most frequently for performing intraprediction on a target block, and the horizontal-up mode that is mode 8shows the highest possibility of being used most infrequently.

According to H.264 standards, a current block is encoded using a totalof 13 modes, that is, 4 modes of the 4×4 intra prediction and 9 modes ofthe 16×16 intra prediction. A bit stream of the current block isgenerated according to an optimal mode among these modes.

However, when some or all values of pixels adjacent to current block donot exist or are not already encoded, it is impossible to apply some orall of the intra prediction modes to the current block. Also, when thereis a large difference between adjacent reference pixels, a differencebetween a prediction block and an original block becomes large.Therefore, the coding efficiency is degraded.

SUMMARY OF THE DISCLOSURE

The present invention is directed to an image encoding apparatus tominimize the amount of coding bits of a residual block.

One aspect of the present invention provides an apparatus including: anintra prediction unit configured to generate a prediction blockaccording to an intra prediction mode selected out of a plurality ofintra prediction modes; a subtracting unit configured to generate aresidual block by calculating a difference between the prediction blockand an original block; a transform unit configured to transform theresidual block to generate a transform block; a quantization unitconfigured to quantize the transform block using a quantization stepsize to generate a quantized transform block; a scanning unit configuredto scan the quantized transform block to generate one-dimensional (1D)quantized coefficients; and an entropy coding unit configured toentropy-code the 1D quantized coefficients, wherein, when a size of thetransform block is 8.times.8, the quantized transform block is dividedinto a plurality of sub-blocks, wherein the plurality of sub-blocks arescanned and coefficients of each sub-block are scanned using a scanpattern, wherein the scan pattern for scanning the plurality ofsub-blocks and for scanning the coefficients of each sub-block isdetermined according to the selected intra prediction mode, wherein thescan pattern for scanning the plurality of sub-blocks is the same as thescan pattern for scanning the coefficients of each sub-block, wherein,when the selected intra prediction mode is a horizontal mode, the scanpattern is a vertical scan, wherein the selected intra prediction modeis coded using an upper intra prediction mode and a left intraprediction mode, and wherein the intra prediction unit includes: areference pixel generating unit configured to generate reference pixelsusing available reference pixels of a current block when referencepixels do not exist; a reference pixel filtering unit configured toadaptively filter reference pixels adjacent to the current block basedon the selected intra prediction mode and size information of thecurrent block; and a prediction block generating unit configured togenerate the prediction block of the current block according to theselected intra prediction mode.

Preferably, when the selected intra prediction mode is one out of aplurality predetermined number of intra prediction modes adjacent to thehorizontal mode, the scan pattern is the vertical scan.

Preferably, the plurality of sub-blocks are scanned in a reversedirection.

Preferably, when the upper intra prediction mode is unavailable, theupper intra prediction mode is set to a DC mode.

Preferably, the quantization step size is determined per coding unit ofa predetermined size.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a moving picture coding apparatusaccording to the present invention.

FIG. 2 is a flow chart illustrating an operation of a scanning unitaccording to the present invention.

FIG. 3 is a block diagram illustrating a moving picture decodingapparatus according to the present invention.

FIG. 4 is a block diagram illustrating an intra prediction unitaccording to the present invention.

FIG. 5 is a conceptual diagram showing positions of reference pixelsused for intra prediction according to the present invention.

FIG. 6 a flow chart illustrating a process of generating referencepixels according to the present invention.

FIG. 7 is a block diagram illustrating an intra prediction unit of amoving picture decoding apparatus according to the present invention.

DETAILED DESCRIPTION OF THE DISCLOSURE

Hereinafter, various embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the exemplaryembodiments disclosed below, but can be implemented in various types.Therefore, many other modifications and variations of the presentinvention are possible, and it is to be understood that within the scopeof the disclosed concept, the present invention may be practicedotherwise than as has been specifically described.

For image coding, each picture consists of one or more slices, and eachslice consists of a plurality of coding units. Since an image of ahigh-definition (HD) grade or higher has many smooth regions, an imagecompression can be improved by encoding the image with coding units ofvarious sizes.

A size of the coding unit according to the present invention may be16×16, 32×32 or 64×64. A size of the coding unit may also be 8×8 orless. A coding unit of the largest size is referred to as a supermacroblock (SMB). A size of SMB is indicated by a smallest size of thecoding unit and depth information. The depth information indicates adifference value between the size of SMB and the smallest size of thecoding unit.

Therefore, the size of coding unit to be used for coding pictures may bea size of SMB or sub-block of SMB. Sizes of coding units are defaultvalues or are indicated in a sequence header.

A SMB consists of one or more coding units. The SMB has a form of arecursive coding tree so as to include the coding units and a divisionstructure of the coding units. When the SMB is not divided into fourcoding units, the coding tree may consist of information indicating thatthe SMB is not divided and one coding unit. When the SMB is divided intofour coding units, the coding tree may consist of information indicatingthat the SMB is divided into four sub-coding trees. Likewise, eachsub-coding tree has the same structure as the largest coding unit (LCU).However, a coding unit of the smallest coding unit (SCU) size is notdivided into coding units.

Meanwhile, each coding unit in the coding tree is subjected to intraprediction or inter prediction in units of the coding unit itself or asub-partition. A unit in which the intra prediction or the interprediction is performed is referred to as a prediction unit. A size ofthe prediction unit may be 2N×2N or N×N for intra prediction. A size ofthe prediction unit may be 2N×2N, 2N×N, N×2N or N×N for interprediction. Here, 2N denotes horizontal and vertical lengths of a codingunit.

A coding unit includes a prediction mode of the prediction unit and sizeinformation on the prediction unit. To improve coding efficiency, theprediction mode and the size information may be combined andjoint-coded. In this case, each coding unit includes a joint-codedprediction type.

A coding unit includes one or more additional information container.Each additional information container contains additional information ofeach prediction unit required for generating a prediction block of eachprediction unit. In intra prediction, the additional informationincludes encoded intra prediction information. In inter prediction, theadditional information includes encoded motion information. The motioninformation includes a motion vector and a reference picture index.

A coding unit also includes a residual signal container for containingencoded residual signals of the coding unit. The residual signalcontainer contains one transform tree, one luminance residual signalcarrier and two chrominance residual signal carriers. The transform treeindicates information indicating a division structure of transform unitsfor the residual signal included in the coding unit. Also, the transformtree includes information indicating whether or not a residual signal ofeach transform unit is 0.

The residual signal container has a form of a recursive coding tree.Therefore, if the coding unit is not divided into four sub-coding unit,the residual signal container contains quantization information andencoded residual signal. If the coding unit is divided into foursub-coding units, the residual signal container contains quantizationinformation and an additional four residual signal containerscorresponding to four sub-coding units. Each additional residual signalcontainer corresponding to each sub-coding unit has the same structureof the residual signal container of the coding unit, but does notcontain the quantization information.

A prediction unit can be divided unequally. It may be more effective tounequally divide an image signal in a specific direction according to aboundary of an image and perform intra or inter prediction compressing aresidual signal.

The simplest method is to divide a coding unit into two blocks using astraight line so as to extract statistical dependency of a predictionregion on local topography. A boundary of an image is matched tostraight line and divided. In this case, dividable directions may belimited to a predetermined number. For example, a method of dividing ablock may be limited to four directions of horizontal, vertical, upwarddiagonal and downward diagonal directions. Also, the division may belimited to the horizontal and vertical directions only. The number ofdividable directions can be three, five, seven and so on. The number ofdividable directions may vary according to a size of the coding block.For example, for a coding unit of a large size, the number of dividabledirections may be relatively increased.

In inter prediction, when one coding unit is divided into two predictionunits for more adaptive prediction, motion estimation and motioncompensation should be performed on each of prediction units. Afterprediction blocks are derived from the respective two prediction unitsdivided from the one coding unit, the two prediction blocks may be addedto generate the prediction block having the same size as the codingunit. In this case, to reduce difference between pixel values on bothsides of a division boundary of the prediction block of the coding unitsize, pixels located at the division boundary may be filtered. Theprediction block may be generated such that the prediction blockscorresponding to the respective prediction units overlap, and theoverlapping boundary portion may be smoothed to generate the oneprediction block.

FIG. 1 is a block diagram illustrating a moving picture coding apparatusaccording to the present invention.

Referring to FIG. 1, a moving picture coding apparatus 100 according tothe present invention includes a picture division unit 110, a transformunit 120 (alternatively referred to herein as a “transformer”), aquantization unit 130 (alternatively referred to herein as a“quantizer”), a scanning unit 131 (alternatively refereed to herein as a“scanner”), an entropy coding unit 140 (alternatively referred to hereinas an “entropy coder”), an intra prediction unit 150 (alternativelyreferred to herein as an “intra predictor”), an inter prediction unit160, an inverse quantization unit 135, an inverse transform unit 125, apost-processing unit 170, a picture storing unit 180, a subtracter 190and an adder 195.

The picture division unit 110 analyzes an input video signal to divideeach LCU of a picture into one or more coding units each of which has apredetermined size, determine prediction mode of each coding unit, anddetermines size of prediction unit per each coding unit. The picturedivision unit 110 sends the prediction unit to be encoded to the intraprediction unit 150 or the inter prediction unit 160 according to theprediction mode. Also, the picture division unit 110 sends theprediction units to be encoded to the subtracter 190.

The transform unit 120 transforms residual signals between an originalblock of an input prediction unit and a prediction block generated bythe intra prediction unit 150 or the inter prediction unit 160. Theresidual block may have a size of coding unit. The residual block may bedivided into optimal transform units and transformed. A transform matrixtype may be adaptively determined according to a prediction mode and anintra prediction mode. The transform unit of the residual signal may betransformed by horizontal and vertical one-dimensional (1D) transformmatrices. In inter prediction, one predetermined transform matrix typeis applied. In intra prediction, there is a high possibility that theresidual signals will have vertical directivity when the intraprediction mode of the current prediction unit is horizontal. Thus, adiscrete cosine transform (DCT)-based integer matrix is applied to thevertical direction, and a discrete sine transform (DST) or KarhunenLo{tilde over (e)}ve transform (KLT)-based integer matrix is applied tothe horizontal direction. When the intra prediction mode is vertical, aDST or KLT-based integer matrix is applied to the vertical direction,and a DCT-based integer matrix is applied to the horizontal direction.Also, in intra prediction, the transform matrix may be adaptivelydetermined according to a size of the transform units.

The quantization unit 130 determines a quantization step size forquantizing transform coefficients of the residual block. Thequantization step size is determined per coding unit of a predeterminedsize or more. The predetermined size may be 8×8 or 16×16. Using thedetermined quantization step size and a quantization matrix determinedby a prediction mode, the transform coefficients are quantized. Thequantization unit 130 uses quantization step sizes of one or moreneighboring coding units to generate a quantization step size predictorof the current coding unit. The quantization unit 130 sequentiallyretrieves coding units in the following scan order; 1) a left codingunit of the current coding unit, 2) an above coding unit of the currentcoding unit, and 3) an above left coding unit of the current codingunit. Then, the quantization unit 130 generates the quantization stepsize predictor of the current coding unit using one or two validquantization step sizes. For example, the first valid quantization stepsize encountered in the scan order may be determined as the quantizationstep size predictor. An average of first two valid quantization stepsize retrieved in the scan order may be determined as the quantizationstep size predictor when two or more quantization step sizes are valid,and one valid quantization step size is determined as the quantizationstep size predictor when only the one quantization step size is valid.When the quantization step size predictor is determined, a differencebetween the quantization step size and the quantization step sizepredictor is transmitted to the entropy coding unit 140.

When a slice is divided into coding units, there may be none of a leftcoding unit, an above coding unit and an above left coding unit of thecurrent coding unit. But, there may be a previous coding unit of thecurrent coding unit in the coding order in the maximum coding unit.Thus, coding units adjacent to the current coding unit and the previouscoding unit may be candidates. In this case, the above scan order may bechanged to the following scan order; 1) the left coding unit of thecurrent coding unit, 2) the above coding unit of the current codingunit, 3) the above left coding unit of the current coding unit and 4)the previous coding unit of the current coding unit. The scan order mayvary, or the above left coding unit may be omitted in the scan order.

The quantized transform block is provided to the inverse quantizationunit 135 and the scanning unit 131.

The scanning unit 131 scans the quantized transform coefficients of thequantized transform block, thereby converting the quantized transformcoefficients into 1D quantized transform coefficients. A coefficientscanning pattern is determined according to the intra prediction mode.The coefficient scanning pattern may also be determined according to thesize of the transform unit.

The inverse quantization unit 135 inversely quantizes the quantizedtransform coefficients. The inverse transform unit 125 restores aresidual block of the spatial domain from the inversely quantizedtransform coefficients. The adder 195 generates a reconstructed block byadding the residual block reconstructed by the inverse transform unit125 and the prediction block from the intra prediction unit 150 or theinter prediction unit 160.

The post-processing unit 170 performs a de-blocking filtering processfor removing blocking artifact generated in a reconstructed picture, anadaptive offset application process for complementing a differencebetween the reconstructed picture and the original image per pixel, andan adaptive loop filter process for complementing a difference betweenthe reconstructed picture and the original image in a coding unit.

The de-blocking filtering process may be applied to a boundary betweenprediction units having a predetermined size or more and betweentransform units. The predetermined size may be 8×8. The de-blockingfiltering process includes a step of determining a boundary to befiltered, a step of determining boundary filtering strength to beapplied to the boundary, a step of determining whether or not to apply ade-blocking filter, and a step of selecting a filter to be applied tothe boundary when it is determined to apply the de-blocking filter.

Whether or not to apply the de-blocking filter is determined accordingto i) whether or not the boundary filtering strength is greater than 0and ii) whether or not a value indicating the difference betweenboundary pixels of P block and Q block is smaller than a first referencevalue determined according to a quantization parameter.

Two or more filters may exist. When an absolute value of a differencebetween two pixels adjacent to the block boundary is equal to or largerthan a second reference value, a weak filter is selected. The secondreference value is determined by the quantization parameter and theboundary filtering strength.

The adaptive offset application process is intended to reduce adifference (distortion) between a pixel subjected to the de-blockingfilter and the original pixel. A picture or slice may be divided into aplurality of offset regions, and an offset mode may be determined perthe offset region. There are four edge offset modes, two band offsetmodes and an offset non-application mode. According to each offset mode,pixels in each offset region are classified into a predetermined numberof classes, and offset corresponding to the classified class is added tothe pixel. In the case of an edge offset mode, a class of a currentpixel is determined by comparing the current pixel value with pixelvalues of two or more pixels adjacent to the current pixel.

The adaptive loop filter process may be performed on the basis of avalue obtained by comparing an original image and a reconstructed imageto which the de-blocking filtering process or the adaptive offsetapplication process is applied. An adaptive loop filter (ALF) isdetected through one Laplacian activity value on the basis of a 4×4block. The determined ALF can be applied to all pixels included in a 4×4block or an 8×8 block. Whether or not to apply an ALF may be determinedaccording to coding units. A size and coefficients of a loop filter mayvary according to each coding unit. A slice header may includeinformation indicating whether or not to apply the ALF to each codingunit, filter coefficient information and filter shape information. Inthe case of chrominance components, whether or not to apply the ALF maybe determined in picture units. Unlike luminance, the loop filter mayhave a rectangular shape.

The picture storing unit 180 receives post-processed image data from thepost-processing unit 170, and stores the image in picture units. Apicture may be a frame or a field. The picture storing unit 180 has abuffer (not shown) capable of storing a plurality of pictures.

The inter prediction unit 160 performs motion estimation using one ormore reference pictures stored in the picture storing unit 180, anddetermines a reference picture index indicating the reference picturesand a motion vector. According to the reference picture index and motionvector, the inter prediction unit 160 extracts a prediction blockcorresponding to a prediction unit to be encoded from a referencepicture selected among a plurality of reference pictures stored in thepicture storing unit 180 and outputs the extracted prediction block.

The intra prediction unit 150 performs intra prediction usingreconstructed reference pixel values within a current picture. The intraprediction unit 150 receives the current prediction unit to bepredictively encoded, selects one of a predetermined number of intraprediction modes, and performs intra prediction. The predeterminednumber of intra prediction modes may depend on the size of the currentprediction unit. The intra prediction unit 150 adaptively filtersreference pixels used to generate the intra prediction block. When someof reference pixels are not valid, it is possible to generate thereference pixels at the invalid positions using one or more validreference pixels.

The entropy coding unit 140 entropy-codes the quantized transformcoefficients from the quantization unit 130, intra predictioninformation received from the intra prediction unit 150, motioninformation received from the inter prediction unit 160, and so on.

FIG. 2 is a flow chart illustrating an operation of the scanning unit131 according to the present invention.

It is determined whether the quantized transform block is divided into aplurality of subsets (S110). The determination is based on a size of thequantized transform block (that is, a size of the current transformunit). If the size of the current transform unit is larger than a firstreference size, the quantized transform coefficients of the quantizedtransform block are divided into a plurality of subsets. The firstreference size is 4×4 or 8×8. The first reference size may betransmitted to a decoder through a picture header or a slice header.

When the quantized transform block is not divided into a plurality ofsubsets, a scan pattern to be applied to the quantized transform blockis determined (S120). The step S120 may be performed prior to the stepS110 or regardless of the step S110.

The quantized coefficients of the quantized transform block are scannedaccording to the determined scan pattern (S130). The scan pattern isadaptively determined according to the prediction mode and the intraprediction mode. In inter prediction mode, only one predetermined scanpattern (e.g., zigzag scan) can be applied. In intra prediction mode, ascan pattern determined according to the intra prediction mode may beapplied. For example, a horizontal scan is applied in a vertical intraprediction mode and a predetermined number of intra prediction modesadjacent to the vertical intra prediction mode. A vertical scan isapplied in a horizontal intra prediction mode and a predetermined numberof intra prediction modes adjacent to the horizontal intra predictionmode. The predetermined number varies according to a number of allowedintra prediction modes of a prediction unit or a size of a predictionblock. For example, if the number of allowable directional intraprediction modes is 16, the predetermined number is 4. If the number ofallowable directional intra prediction modes is 33, the predeterminednumber is 8. Meanwhile, Zigzag scan is applied to non-directional intraprediction modes. A non-directional mode may be a direct current (DC)mode or a planar mode.

If it is determined that the quantized transform block is divided into aplurality of subsets, the quantized transform block is divided into aplurality of subsets (S140). The plurality of subsets consist of onemain subset and one or more residual subsets. The main subset is locatedat an upper left side and covers a DC coefficient, and the one or moreresidual subsets cover region other than the main subset.

A scan pattern to be applied to the subsets is determined (S150). A samescan pattern is applied to all the subsets. The scan pattern isadaptively determined according to the prediction mode and the intraprediction mode. The step S150 may be performed prior to the step S110or regardless of the step S110.

When the size of the quantized transform block (that is, the size of thetransform unit) is larger than a second reference size, thepredetermined scan pattern (zigzag scan) may be applied to the quantizedtransform block. The second reference size is, for example, 8×8.Therefore, the step S150 is performed when the first reference size issmaller than the second reference size.

In inter prediction mode, only one predetermined scan pattern (e.g.,zigzag scan) can be applied to each subset. In intra prediction mode,the scan pattern is adaptively determined as the same as the step S130.

The quantized transform coefficients of each subset are scannedaccording to the scan pattern (S160). The quantized transformcoefficients in each subset are scanned in the reverse direction. Thatis, the quantized transform coefficients are scanned from a lastnon-zero coefficient to other non-zero coefficients according to thescan pattern, and entropy-coded.

The zigzag scan may be applied to scan the subsets. The subsets may bescanned beginning with the main subset to the residual subsets in aforward direction, or can be scanned in the reverse direction. A scanpattern for scanning the subsets may be set the same as a scan patternfor scanning the quantized transform coefficients.

An encoder transmits information capable of indicating a position of thelast non-zero quantized coefficient of the transform unit to a decoder.The encoder also transmits information capable of indicating a positionof the last non-zero quantized coefficient in each subset to thedecoder.

FIG. 3 is a block diagram illustrating a moving picture decodingapparatus according to the present invention.

The moving picture decoding apparatus according to the present inventionincludes an entropy decoding unit 210, an inverse scanning unit 220, aninverse quantization unit 230, an inverse transform unit 240, an intraprediction unit 250, an inter prediction unit 260, a post-processingunit 270, a picture storing unit 280, an adder 290 and a switch 295.

The entropy decoding unit 210 extracts intra prediction information,inter prediction information and quantized coefficients information froma received bit stream. The entropy decoding unit 210 transmits the interprediction information to the inter prediction unit 260, intraprediction information to the intra prediction unit 250 and thequantized coefficients information to the inverse scanning unit 220.

The inverse scanning unit 220 converts the quantized coefficientsinformation into two dimensional quantized transform block. One of aplurality of inverse scan patterns is selected for the conversion. Theinverse scan pattern is selected based on at least one of the predictionmode and the intra prediction mode. An operation of the inverse scanningunit 220 is the same as the inverse operation of the scanning unit 131of FIG. 1. For example, if a size of a transform unit to be decoded islarger than the first reference size, the quantized transformcoefficients of each subset are inversely scanned according to theselected inverse scan pattern to generate a plurality of subsets and aquantized transform block having a size of the transform unit isgenerated using the plurality of subsets.

The inverse quantization unit 230 determines a quantization step sizepredictor of the current coding unit. The operation to determine thequantization step size predictor is same as the procedure of thequantization unit 130 of FIG. 1. The inverse quantization unit adds thedetermined quantization step size predictor and a received residualquantization step size to generate a quantization step size of thecurrent coding unit. The inverse quantization unit 230 restores inversequantized coefficients using a quantization matrix determined by thequantization step size. The quantization matrix varies according to thesize of the current block and prediction mode.

The inverse transform unit 240 inversely transforms the inversequantized block to restore a residual block. The inverse transformmatrix to be applied to the inverse quantized block is adaptivelydetermined according to the prediction mode and the intra predictionmode. The determination procedure of the inverse transform matrix is thesame as the procedure in the transform unit 120 of FIG. 1.

The adder 290 adds the restored residual block and a prediction blockgenerated by the intra prediction unit 250 or the inter prediction unit260 to generate a reconstructed image block.

The intra prediction unit 250 restores the intra prediction mode of thecurrent block based on the received intra prediction information, andgenerates a prediction block according to the restored intra predictionmode.

The inter prediction unit 260 restores reference picture indexes andmotion vectors based on the received inter prediction information, andgenerated a prediction block using the reference picture indexes and themotion vectors. When the motion vector does not indicate a pixelposition, the prediction block is generated using an interpolationfilter.

The post-processing unit 270 operates the same as the post-processingunit 160 of FIG. 1

FIG. 4 is a block diagram illustrating the intra prediction unit 150 ofa moving picture coding unit 100 according to the present invention.

Referring to FIG. 4, the intra prediction unit 150 includes a referencepixel generating unit 151 (alternatively referred to herein as a“reference pixel generator”), a reference pixel filtering unit 152(alternatively referred to herein as a “reference pixel filter”), aprediction mode determining unit 153, a prediction block generating unit154 (alternatively referred to herein as a “prediction blockgenerator”), a prediction block filtering unit 155 and a prediction modecoding unit 156.

The reference pixel generating unit 151 determines that it is necessaryto generate reference pixels for intra prediction, and generatesreference pixels if it is necessary to generate the reference pixels.

FIG. 5 is a conceptual diagram showing positions of reference pixelsused for intra prediction according to the present invention. As shownin FIG. 5, the reference pixels of the current prediction unit used forintra prediction consist of above reference pixels, left referencepixels and a corner reference pixel. The above reference pixels coveringareas C and D are located at (x=0, . . . , 2L−1, y=−1), and the leftreference pixels covering areas A and B are positioned at (x=−1, y=0, .. . , and 2M−1). Here, L is a width of the current prediction block, andM is a height of the current prediction block.

The reference pixel generating unit 151 determines whether the referencepixels are available or not. If one or more reference pixels are notavailable, the reference pixel generation unit 151 generates referencepixels using available reference pixel.

When left reference pixels or the above reference pixels are notavailable, the reference pixels are generated as follows. When thecurrent prediction unit is located at the upper boundary of a picture ora slice, the above reference pixels covering areas C and D do not exist.When the current prediction unit is located at the left boundary of apicture or a slice, the left reference pixels covering areas A and B donot exist. In those cases, reference pixels are generated by copying thevalue of an available pixel closest to the unavailable pixel. That is,when the current prediction unit is located at the upper boundary of apicture or a slice, the above reference pixels can be generated bycopying an uppermost left reference pixel. When the current predictionunit is located at the left boundary of a picture or a slice, the leftreference pixels can be generated by copying a leftmost above referencepixel. The method may vary per sequence, picture or slice if necessary.

When some of the left reference pixels or the above reference pixels arenot available, the reference pixels are generated as follows.

If the unavailable reference pixel exists in only one direction from theavailable pixels, the reference pixel is generated by copying the valueof an available pixel closest to the unavailable pixel. For example,when the current block is located at the right boundary of a picture ora slice or a LCU, the reference pixels covering area D are notavailable. Also, when the current block is located at the below boundaryof a picture or a slice or a LCU, the reference pixels covering area Bare not available. The reference pixels are generated using two or moreavailable pixels closest to the unavailable pixel.

If the unavailable reference pixel exists between the available pixels,the reference pixel is generated using two available reference pixelsclosest to the unavailable pixel in both sides. For example, when thecurrent block is located at the upper boundary of a slice and the aboveleft block of the current block is available, the reference pixelscovering area C are not available, but the reference pixels coveringareas A and D are available. The reference pixel is generated byaveraging the two available reference pixels. But, linear interpolationmay be used to generate the reference when the area covered by theunavailable reference pixels is large.

When all of the reference pixels are unavailable, the reference pixelsare generated by using two or more pixels located inside of theprediction unit. For example, when the current block is located at theleft upper boundary of a picture or a slice, all the reference pixelsare unavailable.

FIG. 6 is a flow chart illustrating a process of generating referencepixels according to the present invention.

The process of generating reference pixels using two pixels is asfollows. A left above pixel (◯) and one of a right above pixel (□), aleft below pixel (Δ) and a right below pixel (Δ) may be used. When theleft above pixel and the right above pixel are used, the referencepixels existing above the left above pixel and the right above pixel aregenerated by copying the left above pixel and the right above pixel andthe copied reference pixels are used to generate reference pixelscovering area C. The reference pixels are generated using an average orlinear interpolation. The reference pixels covering D are generatedusing the right above pixel or a plurality of the generated abovepixels. When the left above pixel and the left below pixel are used, thesame method is applied. When the left above pixel and the right belowpixel are used, the right below pixel is copied in horizontal directionand vertical direction to generate two corresponding reference pixelsand then the residual reference pixels are generated as the same asdescribed above.

The process of generating reference pixels using three pixels is asfollows. A left above pixel, a right above pixel and a left below pixelmay be used. The pixels are copied to generate reference pixelscorresponding to the pixels. The residual reference pixels are generatedas the same as described above.

The values of the pixels used to generate the reference pixels aretransmitted to the decoder. To minimize the amount of bits to betransmitted, the value of the left above pixel and the differencebetween the value of the left above pixel and the value of other pixel.The value of the left above pixel may be a quantized value or beentropy-coded.

When a slice type is I, it is more effective to generate the referencepixels using two pixels.

Alternatively, the reference pixels are generated by copying pixelsexisting same positions of a reference picture. If there are not pixelsexisting same positions, nearest pixels are used to generate thereference pixels.

The reference pixel filtering unit 152 filters reference pixels of thecurrent prediction unit. Low-pass filter is applied to smooth thedifference between adjacent reference pixels. The low-pass filter may bea 3-tap filter [1, 2, 1] or a 5-tap filter [1, 2, 4, 2, 1].

The filter may be adaptively applied according to a size of the currentblock. If the size of the current block is equal to or smaller than apredetermined size, the filter is not applied. The predetermined sizemay be 4×4.

The filter may also be adaptively applied according to a size of thecurrent block and intra prediction mode.

In the horizontal mode or the vertical mode, a prediction pixel isgenerated using one reference pixel. Therefore, a filter is not appliedin the horizontal mode and the vertical mode. In the DC mode, aprediction pixel is generated using average of the reference pixels.Therefore, a filter is not applied in the DC mode because the predictionpixel is not affected by the difference between adjacent pixels.

In the intra prediction mode 3, 6 or 9 having a direction of 45° withreference to the horizontal or vertical direction, a filter is appliedregardless of the size of the prediction unit or applied when thecurrent block is larger than a smallest prediction unit. A first filtermay be applied to a prediction unit having a size smaller that apredetermined size, and a second filter stronger than the first filtermay be applied to a prediction unit having a size equal to or largerthan the predetermined size. The predetermined size may be 16×16.

In the intra prediction modes other than the vertical mode, thehorizontal mode, the DC mode and the intra prediction mode 3, 6 and 9, afilter may be adaptively applied according to the size of the currentprediction unit and the intra prediction mode.

The prediction block generating unit 153 generates a prediction blockcorresponding to the intra prediction mode. The prediction block isgenerated using the reference pixels or linear combination of thereference pixels based on the intra prediction mode. The referencepixels to be used to generate the prediction block may be filtered bythe reference pixel filtering unit 152.

The prediction block filtering unit 154 adaptively filters the generatedprediction block according to the intra prediction mode to minimize theresidual signal between a prediction block and a current block to beencoded. The difference between a reference pixel and a prediction pixeladjacent to the reference pixel varies according to the intra predictionmode. Therefore, filtering of the prediction pixel adjacent to thereference pixel enables the difference to be decreased.

In the DC mode, the difference may be large because the prediction pixelis an average value of the reference pixels. Therefore, the predictionpixels of upper line and left line which are adjacent to the referencepixels are filtered using the reference pixels. The upper leftprediction pixel adjacent two reference pixels is filtered by 3-tapfilter. The other prediction pixels adjacent to one reference pixel arefiltered by 2-tap filter.

In the vertical mode (mode 0), the diagonal down-left mode (mode 6), andthe intra prediction modes 22, 12, 23, 5, 24, 13, 25 between the mode 0and mode 6, a prediction block is generated using only above referencepixels. Therefore, the difference between a reference pixel and aprediction pixel adjacent to the reference pixel may be larger as theposition of the prediction pixel goes down.

Also, in the horizontal mode (mode 1), the diagonal up-right mode (mode9), and the intra prediction modes 30, 16, 31, 8, 32, 17, 33 between themode 1 and mode 9, a prediction block is generated using only leftreference pixels. Therefore, the difference between a reference pixeland a prediction pixel adjacent to the reference pixel may be larger asthe position of the prediction pixel goes right.

Accordingly, some prediction pixels of the prediction block may befiltered for compensate the difference in directional intra predictionmode other than DC mode.

In the mode 6, all or some prediction pixels adjacent to the referencepixel are filtered. The some prediction pixels may be N/2 lower pixelsadjacent to the reference pixel. N is the height of the predictionblock.

In the mode 9, all or some prediction pixels adjacent to the referencepixel are filtered. The some prediction pixels may be M/2 right pixelsadjacent to the reference pixel. M is the width of the prediction block.

In a predetermined number of directional intra prediction modes whichare closer to the mode 6 and exist between mode 0 and mode 6, the samefiltering method as in the mode 6 can be used. The same filtering methodcan be used in the directional intra prediction modes between mode 1 andmode 9.

Meanwhile, the filter is adaptively applied according to the size of thecurrent prediction unit. For example, a filtered is not applied to apredetermined size which is determined by the intra prediction mode.

The prediction block filtering unit 154 may be integrated into theprediction block generating unit 153. Alternatively, the predictionblock is generated by using the combination of the generating operationand the filtering operation.

The intra prediction mode determining unit 155 determines the intraprediction mode of a current prediction unit using reference pixels. Theintra prediction mode determining unit 155 selects one intra predictionmode in which the amount of coding bits of a residual block is minimizedas the intra prediction mode of the current prediction unit. Theprediction block may be generated by filtered reference pixels or may bea block filtered by the prediction block filtering unit 154.

The prediction block transmitting unit 157 transmits the predictionblock generated based on the intra prediction mode to the subtracter190.

The intra prediction mode coding unit 156 encodes the intra predictionmode of the current prediction unit determined by the intra predictionmode determining unit 155. The intra prediction mode coding unit 156 maybe integrated into the intra prediction unit 150 or into the entropycoding unit 140.

The intra prediction mode coding unit 156 encodes the intra predictionmode of the current prediction unit using an above intra prediction modeof the current prediction unit and a left intra prediction mode of thecurrent prediction unit.

First, the above intra prediction mode and the left intra predictionmode of the current prediction unit are derived. When there exist aplurality of above prediction units of the current prediction unit, theplurality of above prediction units are scanned in a predetermineddirection (e.g., from right to left) to determine the intra predictionmode of a first valid prediction unit as an above intra prediction mode.Also, when there exist a plurality of left prediction units of thecurrent prediction unit, the plurality of left prediction units arescanned in a predetermined direction (e.g., from bottom to top) todetermine the intra prediction mode of a first valid prediction unit asa left intra prediction mode. Alternatively, among a plurality of validprediction units, the intra prediction mode of a valid prediction unithaving the lowest intra prediction mode number may be set as an aboveintra prediction mode.

When the above intra prediction mode or the left intra prediction modeis not available, the DC mode (mode 2) may be set to as the above intraprediction mode or the left intra prediction mode. The above intraprediction mode or the left intra prediction mode is treated asunavailable when there does not exist a corresponding prediction unit.

Next, the above intra prediction mode or the left intra prediction modeis converted into one of the permissible modes when the above intraprediction mode number or the left intra prediction mode number is equalto or greater than the number of intra prediction modes permissible forthe current prediction unit. For example, when the size of the currentprediction unit is 4×4, the intra prediction mode of the valid intraprediction mode candidate is converted into one of nine modes (mode 0 tomode 8), and when the size of the current prediction unit is 64×64, theintra prediction mode value of the valid intra prediction mode candidateis converted into one of three modes (mode 0 to mode 2).

Next, if the intra prediction mode of the current prediction unit is thesame as any one of the above and left intra prediction modes, a flagindicating that the intra prediction mode of the current prediction unitis the same as any one of the above and left intra prediction modes anda flag indicating one of the above and left intra prediction modes aretransmitted to the decoder. In this case, if the above and left intraprediction modes are same, the flag indicating one of the above and leftintra prediction modes can be omitted. Also, if only one of the aboveand left intra prediction modes is available, the flag indicating one ofthe above and left intra prediction modes can be omitted.

But, if the intra prediction mode of the current prediction unit is notthe same as any one of the above and left intra prediction modes, theintra prediction mode number of the current prediction unit is comparedwith the above intra prediction mode number and the left intraprediction mode number. A mode change value is counted. The mode changevalue is the number of intra prediction modes having intra predictionmode number that are not greater than the intra prediction mode numberof the current prediction unit. Then, the intra prediction mode of thecurrent prediction unit is changed using the mode change value. Thechanged intra prediction mode is determined as final intra predictionmode information to be transmitted.

A table used for encoding the final intra prediction mode is selectedaccording to whether the above and the left intra prediction modes aresame or not.

FIG. 7 is a block diagram illustrating the intra prediction unit 250 ofa moving picture decoding apparatus 200 according to the presentinvention.

The intra prediction unit 250 according to the present inventionincludes a intra prediction mode decoding unit 251, a reference pixelgenerating unit 252, a reference pixel filtering unit 253, a predictionblock generating unit 254, a prediction block filtering unit 255 and aprediction block transmitting unit 256.

The intra prediction mode decoding unit 251 restores the intraprediction mode of a current prediction unit as follows.

First, the intra prediction mode decoding unit 251 receives additionalinformation included in the additional information container forgenerate a prediction block. The additional information includes aprediction flag and residual prediction information. The prediction flagindicates whether the prediction mode of the current prediction unit isthe same as one of prediction modes of adjacent prediction blocks. Theresidual prediction information includes information determined by theprediction flag. If the prediction flag is 1, the residual predictioninformation may include an index of the intra prediction mode candidate.The index of the intra prediction mode designates the intra predictionmode candidate. If the prediction flag is 0, the residual informationmay include residual intra prediction mode number.

Intra prediction mode candidates of the current prediction unit arederived. The intra prediction mode candidates are derived using intraprediction modes of adjacent prediction units. For example, the adjacentprediction units are an above prediction unit and a left predictionunit. When there are a plurality of above prediction units or aplurality of left prediction units, the intra prediction mode of theabove or left prediction unit is determined as the same as described inthe operation of the intra prediction encoding unit 156 of the encodingapparatus 100. Also, when the mode number of a valid intra predictionmode candidate is equal to or greater than the number of intraprediction modes permissible for the current prediction unit, the validintra prediction mode candidate is converted into one of the permissiblemodes as described in the intra prediction encoding unit 156.

Next, if the received prediction mode indicates that the intraprediction mode of the current prediction unit is equal to one of theintra prediction candidates and the index of the intra prediction modecandidate exists, the intra prediction mode indicated by the index isset as the intra prediction mode of the current prediction unit.

If the received prediction mode indicates that the intra prediction modeof the current prediction unit is equal to one of the intra predictioncandidates and the index of the intra prediction mode candidate does notexist, the one valid intra prediction mode is set as the intraprediction mode of the current prediction unit.

If the received prediction mode indicates that the intra prediction modeof the current prediction unit is not equal to any one of the intraprediction candidates, the intra prediction mode of the currentprediction unit is restored by comparing the residual intra predictionmode number and the mode numbers of the valid intra prediction modecandidates.

The reference pixel generating unit 252 generates reference pixels usingthe same method as described in the reference pixel generating unit 151of the coding apparatus 100. But, the reference pixel generating unit252 may generate reference pixels only when the reference pixels usedfor generating a prediction block and determined by the intra predictionmode are not available.

The reference pixel filtering unit 253 adaptively filters the referencepixels based on the intra prediction mode and a size of the predictionblock. The filtering condition and method are same as those of thereference pixel filtering unit 152 of the coding apparatus 100.

The prediction block generating unit 254 generates a prediction blockusing the reference pixels according to the restored intra predictionmode.

The prediction block filtering unit 255 adaptively filters theprediction pixel according to the restored intra prediction mode. Thefiltering operation is the same as that of the prediction blockfiltering unit 154 of the coding apparatus 100.

The prediction block transmitting unit 256 transmits a prediction blockreceived from the prediction block generator 254 or the prediction blockfiltering unit 255 to the adder 290.

The invention claimed is:
 1. An image encoding apparatus, comprising: anintra predictor configured to generate a prediction block according toan intra prediction mode selected out of a plurality of intra predictionmodes; a subtracter configured to generate a residual block bycalculating a difference between the prediction block and an originalblock; a transformer configured to transform the residual block togenerate a transform block; a quantizer configured to quantize thetransform block using a quantization step size to generate a quantizedtransform block; a scanner configured to scan the quantized transformblock to generate one-dimensional (1D) quantized coefficients; and anentropy coder configured to entropy-code the 1D quantized coefficients,wherein, when a size of the transform block is 8×8, the quantizedtransform block is divided into a plurality of sub-blocks after thequantization of the transform block, wherein the scanner scans theplurality of sub-blocks using a first scan pattern determined accordingto the selected intra prediction mode and scans coefficients of eachsub-block using a second scan pattern determined according to theselected intra prediction mode, wherein the first scan pattern is a scanorder applied to the plurality of sub-blocks, and the second scanpattern is a scan order applied to the coefficients of each sub-block,wherein the first scan pattern is the same as the second scan pattern,wherein, when the selected intra prediction mode is a horizontal mode,the scan pattern is a vertical scan, and wherein the intra predictorincludes: a reference pixel generator configured to generate referencepixels using available reference pixels of a current prediction blockwhen reference pixels do not exist; a reference pixel filter configuredto adaptively filter reference pixels adjacent to the current predictionblock based on the selected intra prediction mode and size informationof the current prediction block; and a prediction block generatorconfigured to generate the prediction block of the current predictionblock according to the selected intra prediction mode.
 2. The apparatusof claim 1, wherein, when the selected intra prediction mode is one outof a predetermined number of intra prediction modes adjacent to thehorizontal mode, the scan pattern is the vertical scan.
 3. The apparatusof claim 1, wherein the plurality of sub-blocks are scanned in a reversedirection.
 4. The apparatus of claim 1, wherein the quantization stepsize is determined per coding unit of a predetermined size.